1. Field of the Invention
The invention concerns a rotor for a gantry of a computed tomography apparatus and a computed tomography apparatus with such a rotor.
2. Description of the Prior Art
Computed tomography apparatuses enable the reconstruction of three-dimensional slice or volume images of an examination region for diagnostic purposes. The reconstruction of an image ensues on the basis of projections of an examination region that are acquired by irradiating a subject with an x-ray fan beam from different projection directions by rotation of an acquisition device, so that measurement data for parallel projections from an angle range of at least 180 degrees plus the fan beam angle are obtained for reconstruction of an image. To produce the rotation of the acquisition device, the computed tomography apparatus has a gantry that has a stationary rotating frame and a rotor mounted so that it can rotate by means of a rotating support device. The acquisition device is mounted on this rotor. The rotor has conventionally been fabricated as a cast part made of an aluminum alloy AlZn10SiMg with a rotor wall in the form of an annular disc and a retention ring running along its outer periphery for mounting the components of the acquisition device. The wall thicknesses of the rotor vary between 15 and 20 mm.
To avoid movement artifacts in the reconstructed image that can arise due to patient or organ movements, it is sought to select the time window for acquisition of the projections required for reconstruction to be as small as possible by the use of high rotation speeds. Rotation speeds of 210 R/min are achieved in current computed tomography apparatuses. However, in the future the rotation speeds should be raised to at least 300 R/min.
Due to a combination of high rotation speed, large rotation radius and high rotation mass, the rotor represents a highly mechanically stressed component that, in addition to the accommodation of the stresses that are incurred, must also insure that the positions of x-ray tubes and detectors is rigidly maintained, since position shifts of the components of more than 0.15 mm can lead to a significant degradation of the image quality.
Significant primary requirements for the rotor of a gantry are accordingly not only a high stability to transfer the forces but also a high rigidity in order to keep deformations of the rotor (and thus the position shifts of the components of the acquisition device) below the allowable limits, given a simultaneously low weight.
An additional thickening of the existing design would be necessary in order to achieve rotation speeds of 300 R/min and more while keeping the same material. The consequence would be a weight increase of the rotor. Components to drive the rotor and the stationary part of the gantry thus would also have to be adapted to the greater weight. This approach has the disadvantage that it leads to a sensitive weight and volume increase of the entire gantry.